1. Field of the Invention
The present invention relates to an electric device for a bicycle and, especially, to an electric device with reduced volume.
2. Description of the Prior Arts
A bicycle is a very common vehicle that is driven only by manpower. To save strength and decrease burden of a biker, some bicycles are installed with an electric propulsion device, and said bicycles are so-called electric bicycles or electric assist bicycles.
The conventional electric propulsion device has a motor, a sensor and a control unit. The motor is mounted nearby a pedal shaft of a bicycle and has a casing, a rotor assembly and a stator assembly. Two ends of the pedal shaft protrude out of the casing. The rotor assembly is mounted securely on the pedal shaft. The stator assembly is mounted securely in the casing. When the stator assembly is electrified, the stator assembly generates a magnetic field, which rotates the rotor assembly with the pedal shaft. The pedal shaft rotates a chain wheel and a rear wheel of the bicycle to move the bicycle. The sensor measures a rotation rate and torque of the pedal shaft. The control unit is mounted in the casing and has a main circuit board and a rotor detecting circuit board. The main circuit board is electrically connected to the sensor and the stator assembly. The rotor detecting circuit board is electrically connected to the main circuit board. A normal of the rotor detecting circuit board is parallel to an axis of the rotor assembly, and the rotor detecting circuit board is adjacent to the rotor assembly. A magnetic loop is mounted axially on the rotor assembly. The magnetic loop is magnetized to have multiple S magnetic poles and multiple N magnetic poles. The S magnetic poles and the N magnetic poles are annularly staggered, and magnetic fields of the S magnetic poles and the N magnetic poles extend axially. The rotor detecting circuit board detects one magnetic pole among the S magnetic poles and the N magnetic poles. The magnetic field of the stator assembly is changed depending on whether the rotor detecting circuit board detects the S magnetic pole or the N magnetic pole such that the rotor assembly actuates the pedal shaft.
When a biker pedals to rotate the pedal shaft, the sensor measures the rotation rate and the torque of the pedal shaft and then transmits the rotation rate and the torque to the main circuit board. Then, the main circuit board changes the rotation rate of the rotor assembly to match the rotation rate of the pedal shaft in order to integrate a force generated by the motor and a force generated by the biker to decrease the resistance force that the biker encounters.
However, the conventional electric propulsion device has the following shortcomings.
First, the main circuit board and the rotor detecting circuit board of the control unit are mounted in different positions and thus must be connected with each other by flat cables. Then, the casing of the motor must be big enough to accommodate the flat cables. Furthermore, the big casing may interfere with the installation of a bicycle frame. If the casing extends backward too much, the casing even interferes with the rear wheel, such that the bicycle frame must be prolonged or the motor must be minimized in outer diameter. However, minimized outer diameters of the rotor assembly and the stator assembly also lower the output power of the motor.
Second, the main circuit board of the control unit has multiple power switch devices vertically mounted on a surface of the main circuit board. Because the power switch devices are erected upright on the main circuit board, the casing of the motor must be big enough to accommodate the power switch devices, which causes the same shortcoming.
Third, the control unit further has multiple heatsinks to dissipate heat of the power switch device. Each conventional heatsink has an outside part and an inside part. The outside part is installed outside the casing. The inside part protrudes inside the casing to contact the power switch devices. Because the heatsink is an individual component installed on the casing by some other components such as screws, the overall volume of the motor is therefore enlarged. On the other hand, to accommodate the inside part, the casing must be big enough as well. As a result, the conventional heatsink also has the same shortcoming of a large volume.
Further, a bicycle with the electric propulsion device is usually installed with a display unit and various kinds of peripheral devices, such as a power start device, a power adjustment device, power-off brakes and a switch grip shift. The display unit is connected to the peripheral devices to display the information of the peripheral devices, or the biker can control the peripheral devices by operating the display unit.
However, the conventional display unit is connected to the peripheral devices and the main circuit board by analog cables. Each peripheral device is first connected to the display unit by an analog cable, and, then, the display unit is connected to the main circuit board by another analog cable. In addition, the display unit itself is also connected to the main circuit board by another analog cable, and the multiple cables are often disorganized and in disorder. For example, if the bicycle has four peripheral devices, the four peripheral devices are connected to the display units via four analog cables, and the display unit is connected to the main circuit board via five analog cables. As a result, there are nine cables in total, which easily cause disorder of the cables and inconvenience in maintenance.
To overcome the shortcomings, the present invention provides an electric device for a bicycle to mitigate or obviate the aforementioned problems.